Hydrometeor Lofting in Banded Snowfall Events

Mesoscale snow bands in extratropical winter storms produce major societal impacts, and are known to present difficult forecasting challenges.  The synoptic-scale and mesoscale environments of banded snowfall events have been studied extensively; strong mid-level frontogenesis accompanied by weak moist symmetric stability have been identified as important environmental characteristics.  Several recent observational and numerical studies document the occurrence of upward vertical velocities in excess of 1 m s^-1 in these winter storms, a value that roughly corresponds to the fall velocity of snow, and is consistent with the environmental characteristics noted above.  This suggests that lofting of snow could play a significant role in the spatial distribution of snow fall rate and accumulation.  It is interesting that strong ascent, while required for lofting, also increases riming and terminal snow fall velocity, indicating that competing processes are at work.  A recent study of lake-effect snow predictability by Reeves and Dawson (2013) documents the sensitivity of the snowfall distribution to characteristics of microphysics schemes in the Weather Research and Forecasting (WRF) model.  They found that lofting, suspension, and horizontal transport of snow varied with riming rate and terminal fall velocity configurations in several popular microphysics schemes.

Two strategies are employed to test the hypothesis that hydrometeor lofting exerts a significant influence on snowfall distributions.  First, diagnostic variables were added to WRF within the Thompson microphysics scheme, allowing direct output of the mass-weighted fall velocity of snow, and also the vertical snow flux.  These diagnostics demonstrate the degree to which snow is lofted in banded winter storms, and the diagnostics themselves hold potential operational value.  Additionally, experiments were conducted in which the terminal fall velocity of snow was altered (increased and decreased below the default value).  These experiments allow an evaluation of the extent to which lofting contributes to heterogeneity in surface snowfall accumulation.  Preliminary results demonstrate the presence of upward snow flux (lofting) in two winter storm events that featured banded snowfall.